The present invention relates to a thick-film thermal printhead.
An example of a conventional thick-film thermal printhead is shown in FIG. 5 and FIG. 6. Each of these conventional thermal printheads (indicated by reference code P) comprises a rectangular head substrate 1xe2x80x2 and a print substrate 2xe2x80x2, As shown in FIG. 5, the head substrate 1xe2x80x2 has a first longitudinal edge 1axe2x80x2 and a second longitudinal edge 1bxe2x80x2 extending in parallel to each other. Further, the head substrate 1xe2x80x2 has a first end 1cxe2x80x2 and a second end 1dxe2x80x2 extending between the first and the second longitudinal edges. Likewise, the print substrate 2xe2x80x2 has two longitudinal edges and two ends.
The head substrate 1xe2x80x2 has an upper surface entirely covered by a glaze layer 10xe2x80x2 (FIG. 6) made of amorphous glass. On an upper surface of the glaze layer 10xe2x80x2, a linear heating resistor 11xe2x80x2 extending along the first longitudinal edge 1axe2x80x2 is formed. The head substrate 1xe2x80x2 is further formed with a common electrode 12xe2x80x2 and a plurality of individual electrodes 13xe2x80x2. As shown in FIG. 5, the common electrode 12xe2x80x2 extends along the first end 1cxe2x80x2, the first edge 1axe2x80x2, and the second end 1dxe2x80x2. Further, the common electrode 12xe2x80x2 has a plurality of comb-like teeth 12Axe2x80x2 extending in parallel to each other. Each of the comb-like teeth 12Axe2x80x2 has a tip potion 12axe2x80x2 contacting the heating resistor 11xe2x80x2.
Each of the individual electrodes 13xe2x80x2 has a first end portion 13axe2x80x2 and a second end portion 13bxe2x80x2 away therefrom. The first end portion 13axe2x80x2 contacts the heating resistor 11xe2x80x2 and extends between two adjacent comb-like teeth 12Axe2x80x2 On the other hand, the second end portion 13bxe2x80x2 is formed with a bonding pad 13cxe2x80x2. The bonding pad 13cxe2x80x2 is electrically connected to a drive IC 14xe2x80x2 via a connecting wire Wxe2x80x2.
With the above constitution, the heating resistor 11xe2x80x2 is divided into a plurality of regions 15xe2x80x2 by the comb-like teeth 12Axe2x80x2. (FIG. 5 shows only one region 15xe2x80x2.) In each of the regions 15xe2x80x2, electric current is passed selectively via the drive IC 14xe2x80x2, to heat the selected region 15xe2x80x2, making each of the regions 15xe2x80x2 function as a heating dot.
The prior-art thick-film thermal printhead P as described above has a following disadvantage: Specifically, the thermal printhead P can provide a good printing result if the printing is performed at a speed of about 2 inches per second (2 ips). However, if the printing speed is increased to about 6 ips for example, printed image can be partially blurred, or an unintended whisker-like projection (feathering) can be printed on a printing sheet.
A thick-film thermal printhead provided by a first aspect of the present invention comprises: an oblong rectangular substrate having at least one longitudinal edge; a partial glaze layer provided on the substrate along the longitudinal edge; a linear heating resistor formed on the partial glaze layer; a common electrode formed on the substrate and electrically connected to the heating resistor; and a plurality of individual electrodes formed on the substrate and electrically connected to the heating resistor.
According to a preferred embodiment, the partial glaze layer has an arcuate cross section. Further, the partial glaze layer has a thickness of 10-25 xcexcm and a width of 400-1000 xcexcm.
Preferably, the common electrode includes a plurality of comb-like teeth each including a tip portion having a smaller width and a base portion having a larger width.
The tip portion of each comb-like tooth may be entirely formed on the partial glaze layer. In this case, preferably, the base portion of each comb-like tooth is formed only partially on the partial glaze layer.
Preferably, the base portion of each comb-like tooth is spaced from the heating resistor.
Preferably, the base portion of each comb-like tooth extends on both of the partial glaze layer and the substrate.
According to the preferred embodiment, each of the individual electrodes includes a tip portion having a smaller width for contact with the heating resistor, and an intermediate portion having a larger width.
Preferably, the intermediate portion of each individual electrode is spaced from the heating resistor.
Preferably, the intermediate portion of each individual electrode extends on both of the partial glaze layer and the substrate.
Other object, characteristics and advantages of the present invention will become clearer from an embodiment to be described with reference to the attached drawings.